1. Field of the Invention
The invention relates to sensors for determining a fluid pressure. More particularly, the invention relates to capacitive pressure transducers producing an electrical signal indicative of a sensed fluid pressure.
2. Description of the Related Art
Capacitive pressure transducers generally work by sensing the change in capacitance upon deflection of at least one diaphragm, including in its construction conductive metal comprising an electrode, with respect to another electrode. The deflecting diaphragm is coupled to the fluid and moves from a first position to a second, different, position with respect to the other electrode in response to a change in pressure. Such pressure transducers have many advantages, including simplicity, low cost, and reliability.
Examples of pressure transducers of this type are described in U.S. Pat. Nos. 4,227,419, granted Oct. 14, 1980; 4,388,688, granted Jun. 14, 1983; 4,398,426, granted Aug. 16, 1983; 4,617,607, granted Oct. 14, 1986; 5,275,054, granted Jan. 4, 1994; 5,315,877, granted May 32, 1994; 5,329,819, granted Jul. 19, 1994; 5,349,865, granted Sep. 27, 1994; and 5,553,502, granted Sep. 10, 1996. All of these patents are assigned to the assignee of the present invention.
In a typical construction, a diaphragm formed of low mechanical hysteresis insulating material, such as a glass or a ceramic material, such as alumina for example, is employed. When pressure is applied to the diaphragm it flexes toward an adjacent base member, normally of the same or a similar material. As mentioned, layers of conductive material, such as gold metal, form electrodes on facing surfaces of the diaphragm and base member. The base member compares a substrate supporting a layered construction including a peripheral spacer, or glass frit, between the substrate and the diaphragm. As the diaphragm moves in response to a change in pressure of a fluid, the spacing between conductive layers forming electrodes on facing surfaces is changed, thereby changing the capacitance between the two conductive-layer electrodes. Circuitry included with or electrically connected to the transducer converts the capacitance to an electrical signal varying linearly with the pressure sensed, and thereby indicates pressure and pressure changes.
However, use of such a pressure transducer with a fluid having a relatively high dielectric constant, such as water, or even under certain circumstances a vapor of such a fluid, such as water condensation from moist air, can pose problems for this type of sensor. As measured capacitance is the parameter used to give an indication of pressure, the accuracy of the sensor can be affected by changes in capacitance from causes other than deflection of the diaphragm plate due to pressure or change in pressure. If a dielectric fluid, such as water, for example, or even a vapor of such a fluid, can migrate into the sensor to a point that it changes the capacitance by virtue of its presence alone, accuracy will be adversely affected.
For convenient reference, water will be used as an example of a problematic fluid. The dielectric constant of air has a value of just over 1, while that of water is about 80, at room temperature. As will be appreciated, even a small amount of water can affect capacitance, as the energy used to align the water molecules in proximity to the oppositely charged electrodes is a direct loss to the capacitor. However, this use of water is for purposes of exemplary illustration only, and the invention is not limited to a particular problematic fluid.